The principal frame design for automobile frame members is of the "box" type construction for strength and load bearing purposes. These frame members often have great variation in both the horizontal and the vertical profile. The cross-section of such tube members also often varies rather extremely from approximately a square cross-section, to a rectangular cross-section to a round cross-section to a severely flattened cross-section, and to any irregularly shaped combination of the above. While some simple, large radiused profiles with varying cross-sections have been obtained by cold or heat forming a generally cylindrical tube blank, most current manufacturing methods produce the complex-shaped box section tube member by fabricating two "U" section stampings which are then welded together to form the finished part. Unfortunately, material and labor consumption in these processes is enormously inefficient. Also, evidence appears to show a significant noise level reduction where the box section frame member is formed from a tubular blank rather than the welded, double "U" section stampings.
The general operations of bending, stretching, depressing and radially expanding a tube blank, with or without a mandrel, are known. For the majority of metals, it is fairly easy to bend small diameter tubing into an arc having a large radius. But as the diameter of the tubing increases and the radius about which it is to be bent decreases, the tube bending process requires some combination of compression at the inner bending radius of the tube and stretching at the outer radius. Although the outer bending surface of the tube may be stretched to the full extent of the materials rated elongation characteristics, cannot satisfactorily bend a tube with a given diameter about a relatively small bending radius without encountering severe buckling at the inner bending surface or undesirable deformation at the outer bending radius. Some have achieved bending tubes with a certain diameter about relatively small bending radii by controllably dimpling or allowing controlled rippling of the inner tube surface thereby creating less stretching of the outer tube surface.
Other examples of methods for bending a tube are shown in U.S. Pat. No. 4,704,886 shows internally pressurizing a tube blank, gripping the opposite ends of the blank and applying longitudinal tension at the ends while applying a lateral force against the blank to bend the blank. U.S. Pat. No. 4,567,743 shows depressing regions of the tube blank and then expanding the blank within a complementary shaped cavity formed by a pair of dies. U.S. Pat. No. 4,829,803 discloses forming a box-like frame member by internally-pressurizing a preformed tubular blank, closing a pair of die halves around the blank, to partially deform the blank within mating die cavities, and then increasing the internal pressure to exceed the yield limit of the wall of the blank to expand the blank into conformity within the mating die cavities. In the '803 patent, the planar mating surfaces of the die halves are perpendicular to the side walls of the cavities, and the patent teaches that by providing a certain internal pressure to the tubular blank, upon closing the die sections, the blank will spread evenly throughout the cavity and will not be pinched between the closing, mating surface portions of the dies. This procedure may prove satisfactory where the width of the die cavities is much greater than the height. However, where the height of the die cavities is much greater than the width, as is often the case with complex-shaped, box section frame members, the tubular blank will simply not be pushed into the deep recesses of the cavity without pinching between the mating die halves.
What is needed is an apparatus which will form a tubular blank into a box section frame member having variations in the vertical and horizontal profile and in the cross-sectional configuration.